1. Field of the Invention
This invention relates to curable polyurethane systems, which are generally comprised of a polyurethane prepolymer, a catalyst, and a stabilizer for the system. The systems may generally be cured, or polymerized, by treatment with water, such as through exposure to moisture in the air or by immersion in water. This invention also relates to specific applications of these systems.
2. Description of the Prior Art
Polyurethanes have long been known as excellent high polymer plastics which, owing to the wide variety of characteristics that can be introduced into the polymer by control of the chemical nature of the systems, has been employed in extremely varied applications. Among applications that may be mentioned are paints and varnishes, coatings, construction and constructional materials, bearings and friction parts, etc. One particular application that has received increased attention in recent years is the use of polyurethane polymers carried on a flexible, pliable or drapeable fabric which may be arranged about a limb or other portion of a patient's body, and thereafter cured to form a cast or splint. This type of material enjoys a large number of advantages over prior art materials such as Plaster of Paris, in that the material is lightweight, water resistant, may be made porous, is X-ray transparent, etc.
One exemplary medical cast or splint or similar orthopedic device is disclosed and claimed in U.S. Pat. No. 4,427,002. The polyurethane system employed therein is comprised of a prepolymer which is the reaction product of an isocyanate and a polyol, together with a catalyst and a stabilizer and an antifoamant. The catalyst is provided to ensure relatively quick curing times upon immersion of the carrier fabric saturated with this system in water, such that the cast "sets", or becomes rigid, in less than about 15 minutes.
The stabilizer is introduced to prevent the catalyst from auto-catalyzing alternative reactions to the polyurethane polymerization. One alternative reaction, is the allophanate reaction which is well known in the art, and together with isocyanate trimerization is primarily responsible for the premature solidification or polymerization of polyurethane systems which are intended to be stored for substantial periods of time prior to use. The use of a catalyst in a polyurethane prepolymer system complicates the problems of storage stability, in that catalyst which catalyze the urethane polymerization reaction also tend to catalyze these alternative reactions. As these reactions proceed in the absence of water, such products generally have no appreciable storage life.
The stabilizer is introduced to the system in order to bind to the catalyst, or prevent the catalyst from catalyzing the undesirable reaction before the system is immersed in water. The water immersion frees the catalyst to activate the polyurethane polymerization, thus resulting in rapid curing. In the above-referenced U.S. Pat. No. 4,427,002, the entire disclosure of which is incorporated-herein-by-reference, the stabilizer is preferably identified as a mineral acid.
Another reference directed to orthopedic casting materials is U.S. Pat. No. 4,411,262. A wide variety of isocyanate/polyol reaction product systems are disclosed therein, and the reference identifies the use of certain catalysts in conjunction with those prepolymers to improve the cure times secured. The reference does not specifically discuss the use of any stabilizer.
Another recent document discussing orthopedic casting materials comprised of a polyurethane prepolymer system-saturated carried material in U.S. Pat. No. 4,433,680. The particular catalyst employed in the reference is dimorpholinodiethylether. The particular discovery leading to that patent was the observation that the catalyst, which had been used in polyurethane systems for some time, gave both extended shelf life (i.e., did not excessively catalyze the side reactions) while, at the same time, gave excellent cure times upon water immersion (sufficiently catalyzed the water-isocyanate, or urethane, reaction). The reference identifies the use of benzoyl chloride as a stabilizing agent. Column 3, lines 59-60.
Despite the above-referenced attempts to provide a catalyzed, stabilized polyurethane prepolymer system, there remains a need to find a stabilizing agent capable of preventing premature polymerization in a broad range of polyurethane applications. Additionally, there continues to be a need for a catalyst and stabilizer system that will provide sufficient polyurethane cure times for the orthopedic applications discussed above, and yet yield good shelf life and stability in excess of the dimorpholinodiethylether previously employed.